1. Field of the Invention
This invention relates to a polypropylene resin for high-rigidity molded products. More particularly it relates to a polypropylene resin (hereinafter often abbreviated to polypropylene) yielding high-rigidity molded products even without adding any particular additive.
2. Description of the Prior Art
Polypropylene is superior in heat resistance, chemical resistance and electric properties, and also good in rigidity, tensile strength, optical characteristics and processability. Thus it has been broadly used for injection molding, film- or sheet-extrusion molding, blow molding, etc.
However, these properties have been not always satisfactory depending on use applications, and its uses have been restricted.
Particularly the rigidity of polypropylene is lower than those of polystyrene and ABS resin and this has caused a serious bottleneck in broadening its application fields. If its rigidity is improved, it is possible to reduce the thickness of the resulting molded product as much. This is not only effective for resources-saving, but also the cooling velocity at the time of molding is increased; hence it is possible to make the molding velocity per unit time faster and improve the productivity.
The present invention is based on a finding that high-rigidity molded products of polypropylene which have never been obtained can be obtained by using a polypropylene produced by polymerizing propylene so as to develop specified physical properties.
As a known art for improving the rigidity of crystalline polypropylene, for example, there is a process of adding an organic neucleus-creating agent such as aluminum para-t-butylbenzoate, 1,3- or 2,4-dibenzylidenesorbitol, etc. to polypropylene and molding the mixture, but the process has such drawbacks that the cost is high and hence uneconomical, and moreover, the luster, impact strength, tensile elongation, etc. are greatly reduced. As another means for rigidity improvement, there is a process of using various inorganic fillers such as talc, calcium carbonate, mica, barium sulfate, asbestos, calcium silicate, etc., but this process has such drawbacks that the light weight property and transparency specific of polypropylene are not only harmed, but also the impact strength, luster, tensile strength, additive property, etc. are lowered. As a technique of using polypropylene having a higher isotacticity for obtaining high-rigidity molded products (Japanese patent application laid-open No. Sho 55-81125), but polypropylene used therein has an isotacticity in the range of those according to conventional art; hence the effectiveness of improving the rigidity of molded products is still insufficient.
In view of the present status of the abovementioned known art, the present inventors have made strenuous studies for obtaining a polypropylene resin from which high-rigidity molded products can be processed without adding any additive, and as a result have found that when a polypropylene resin which satisfies the requirements of the present invention mentioned below is processed, the objective high-rigidity molded products are obtained.
As apparent from the above description, an object of the present invention is to provide a polypropylene resin from which high-rigidity molded products can be produced. Another object of the present invention is to broaden concrete application fields of polypropylene resins.